Polyphenylene ether resin has good thermal resistance, high impact strength, and good dimensional stability. Accordingly, the resin has been widely applied for common uses. However, the disadvantage could be observed that the polyphenylene ether resin has poor processability due to its excessively high thermal resistance. For this reason, the polyphenylene ether resin is blended with a rubber-modified styrenic resin or a styrenic resin to improve processability for use of electronic appliances such as personal computers, facsimiles, and the like. Further, in case that a blend of a polyphenylene ether and a rubber modified styrene-containing resin is applied to the fields of electric or electronic goods, flame-retardant property should be given to the resin.
A widely known method for giving the flame retardant property is that a halogen-containing compound or an antimony-containing compound is added to a rubber modified styrene-containing resin. However, the disadvantages could be observed that the halogen-containing compound results in the corrosion of the mold itself by the hydrogen halide gases released during the molding process and is fatally harmful due to the toxic gases liberated in case of fire.
Especially, since polybromodiphenyl ether, mainly used for a halogen containing flame retardant, can produce toxic gases such as dioxin or furan during combustion, flame-retardants that are prepared without a halogen-containing compound have become a major concern in this field.
It is commonly a known method to apply a phosphorus or nitrogen compound as halogen-free flame retardant to a resin composition. However, usage of only a phosphorus compound deteriorates heat resistance of a rubber modified styrene-containing resin and does not impart sufficient flame retardant property.
Generally, when a rubber modified styrene-containing resin such as ABS is burned; a char is not produced due to decomposition and vaporization in most parts (Journal of Applied Polymer Science, 1998, vol 68, p 1067). Therefore, to impart an effect of flame retardancy, it is necessary to add a char-forming agent such as polycarbonate or polyphenylene ether resin to a resin composition, which plays a role in prohibiting entrance of oxygen and emission of fuel by forming char on the surface of rubber with three-dimensional carbon bonds under combustion.
Aromatic phosphoric ether is commonly used to give the flame retardancy to the blend.
Japanese Patent Laid-open No. 7-48491 discloses a flameproof thermoplastic resin, made of adding a novolac phenolic resin as a char-forming agent and a phosphoric acid ester as a flame retardant into a thermoplastic copolymer resin composed of a rubber copolymer and an aromatic vinyl monomer. It was found that in order to obtain a good property of flame retardancy, a phenolic resin as a char-forming agent and a phosphoric acid ester compound as a flame retardant should be added in a lot of amount. However, in this case, the heat resistance of the resin composition is dropped suddenly.
In a blend of a polyphenylene ether and a rubber modified styrene-containing resin, the more the rubber modified styrene-containing resin is used, the lower the flame retardancy of the resin composition is getting. Thus, flame retardancy is almost lost when the rubber modified polystyrene is added more than the critical amount. If the amount of acrylonitrle-butadiene-styren copolymer or rubber modified polystyrene resin is increased in the blend of a polyphenylene ether and a rubber modified styrene-containing resin, a great amount of decomposition is produced from combustion, so that a char film is destroyed to decrease flame retardancy of the resin.
U.S. Pat. No. 3,639,506 discloses a resin composition using triphenyl phosphate (TPP) and a halogen-containing compound together as flame retardants for giving flame retardancy to a polyphenylene ether resin and a stryrenic resin. U.S. Pat. No. 3,883,613 discloses a resin composition using trimesityl phosphate as a flame retardant to a polyphenylene ether resin and a stryrenic resin. U.S. Pat. No. 4,526,917 discloses a resin composition using TPP and trimesityl phosphate as flame retardants.
Accordingly, the present inventors have developed a thermoplastic flame retardant resin composition which has a good appearance, flame-retardancy, and impact strength by adding acrylonitrile-rubber-styrene copolymer containing 0.1 to 8% by weight of acrylonitrile in the polystyrene resin excluding rubber to a blend of a rubber modified polystyrene resin and polyphenylene ether resin.